1. Technical Field
The present invention relates generally to transmitting data over asynchronous transfer mode (ATM) networks. More specifically, the present invention relates to preventing frame loss in route switched ATM networks.
2. Description of the Related Art
Route switched networks refer to networks where routing and forwarding decisions are distributed to the edge of a switched/bridged local area network (LAN) infrastructure. Network 100 shown in FIG. 1 is an example of a route switched network. In network 100, the edge devices (e.g., LAN switches 110 and 112) bridge traffic between legacy LAN ports and emulated LAN (ELAN) ports on ATM network 124. In addition, Next Hop Resolution Protocol (NHRP) Client functions are included in legacy LAN stations 102 and 104, with MAC addresses MAC.sub.-- a and MAC.sub.-- b, respectively. This arrangement of devices allows intersubnet shortcut data direct virtual channel connections (shortcut VCCs) to be established. Establishing a shortcut VCC allows the edge devices to utilize their bridging capabilities along with the high-speed switching characteristics of ATM network 124 to create efficient layer-2 connections between stations resident on different layer-3 subnets. Operating in this manner allows for routers in network 100 to be removed from the steady-state data paths and allows the maximum benefits of the switched ATM network to be realized.
When LAN station 102 wishes to establish a shortcut VCC to send information to LAN station 104, LAN station 102 issues a NHRP Resolution Request to determine the MAC address associated with the internet protocol (IP) address of LAN station 104. In network 100, the IP address of LAN station 104 is 9.1.3.1 (while the IP address of LAN station 102 is 9.1.1.1). LAN station 102 sends this NHRP Resolution Request along the routed path to NHRP server 116.
When NHRP server 116 receives the NHRP Resolution Request, NHRP server 116 forwards the Resolution Request to NHRP server 118. NHRP server 118 returns to NHRP server 116 a NHRP Resolution Reply that contains the MAC and ATM addresses associated with LAN station 104. NHRP server 116 then registers the MAC-to-ATM address mapping with LAN emulation server/broadcast and unknown server (LES/BUS) 114, and returns a NHRP Resolution Reply to LAN station 102. This Reply contains the MAC address associated with the IP address of LAN station 104.
LAN station 102 then uses the received MAC address of LAN station 104 as the destination MAC address of frames transmitted to LAN station 104. Information sent from LAN station 102 to LAN station 104 is then delivered via the normal layer 2 procedures. These procedures include LAN switch 110 issuing a LAN emulation address resolution protocol (LE.sub.-- ARP) Request for the ATM address associated with LAN station 104. In response to the LE.sub.-- ARP Request, LES/BUS 114 responds with a LE.sub.-- ARP.sub.-- RESPONSE containing the ATM address of LAN switch 112. LAN switch 110 then initiates signaling to set up a shortcut VCC to the ATM address of LAN switch 112. After the signaling process is finished, frames destined for the MAC address of LAN station 104 are delivered to LAN station 104 via LAN switch 112 over this shortcut VCC.
Route switched network 100 works well after the shortcut VCC is established. However, frames can be lost before the shortcut VCC has been established due to any one of several potential problems. First, some edge devices will not issue a LE.sub.-- ARP.sub.-- REQUEST for a given destination MAC address until a frame has been received from an ELAN with the destination MAC address as the source MAC address (i.e., until the bridge has learned that the MAC address resides on a particular ELAN). In network 100 LAN switch 110 may not issue a LE.sub.-- ARP.sub.-- REQUEST for LAN station 104 on ELAN 112 until it receives a frame from ELAN 112 with the source MAC address of LAN station 104. Not all edge devices behave in the manner described above, however, some have been programmed not to issue a LE.sub.-- ARP.sub.-- REQUEST in the above situation to avoid generating unnecessary traffic on the ATM network.
Another problem can occur during the interval after the destination MAC address has been returned to the NHRP client, but before the shortcut VCC has been established. During this interval, any frames transmitted by the source LAN station to the destination MAC address will be forwarded to the LAN emulation broadcast and unknown server (BUS) by the edge device. However, these frames will not be delivered to the correct destination. In network 100, frames transmitted to the MAC address MAC.sub.-- b by LAN station 102 will be forwarded to LES/BUS 114 by LAN switch 110 if the shortcut VCC has not yet been established. LES/BUS 114 will broadcast these frames to all stations on ELAN 112, but, LAN station 104 will not receive these frames.
Another problem arises when the shortcut VCC has been established and the edge device tries to implement the LAN emulation flush protocol when switching from the BUS data path to the shortcut VCC. The flush protocol will fail since the destination ATM device is not resident on the ELAN and therefore cannot respond to the LE.sub.-- FLUSH.sub.-- REQUEST. As a result, the edge device will wait for the Path Switching Delay period (which is a LAN emulation client configuration parameter that defaults to 6 seconds) before using the newly established shortcut VCC. Frames transmitted during this interim 6 second period may be discarded by the edge device. In network 100, when the shortcut VCC has been established, LAN switch 110 may send a LE.sub.-- FLUSH.sub.-- REQUEST to LES/BUS 114 with the ATM address of LAN switch 112 as the target ATM address. LES/BUS 114 will broadcast the LE.sub.-- FLUSH.sub.-- REQUEST message to ELAN 112, but no station on ELAN 112 will reply. Consequently, LAN switch 110 will not transmit any frames destined for MAC.sub.-- b for a period of time equal to the Path Switching Delay. Frames transmitted during this interval to MAC.sub.-- b by LAN station 102 may be discarded by LAN switch 110.
The first problem is more serious than the next two because it creates a situation where a shortcut VCC will never be established. The second and third problems are not as serious, since they are limited to the initial stages of the shortcut VCC establishment. However, these problems are undesirable and detract from the value of a route switched network.
Thus, it would be desirable to provide a route switch network which did not suffer from any of the three problems mentioned above. In such a network, edge devices would issue a LE.sub.-- ARP.sub.-- REQUEST for a given destination MAC address regardless of whether a frame has been received from an ELAN with that MAC address as the source MAC address. Also, the improved route switched network would ensure that frames were transmitted to the correct destination during the interval after the destination MAC address has been returned to the NHRP client, but before the shortcut VCC had been established. Finally, this network would not delay edge devices for the Path Switching Delay period before using a newly established shortcut VCC.